Method for three-dimensional modeling of the skull and internal structures thereof

ABSTRACT

A method is disclosed in this publication for modeling different internal structures of a head, such as different parts of the brain, the method comprising the step of determining the location of the internal structures, such as the different cerebral parts, of at least one first head in a three-dimensional space by techniques such as magnetic resonance imaging or computer-aided tomography. According to the invention, the external dimensions of at least one second head are determined, and the location data of the internal structures of the first head are scaled in a three-dimensional space to correlate with the external dimensions of the second head, whereby the location data of the internal structures of the second head also become modeled without the need for anatomical images of the second head.

The invention relates to a method according to the preamble of claim 1for three-dimensional modeling of the skull and internal structuresthereof.

This patent application describes a method suited for modifying a set ofmagnetic resonance images taken from the head (standard head) so thatthe external shape of the head determined from such a set of images canbe transformed to correlate with the contour of the head of another testperson or patient. The present method is particularly suited to themagnetic stimulation of the brain as well as to electro-encephalographyand magnetoencephalography.

In transcranial magnetic stimulation (TMS) of the brain, a coil excitedwith a strong current pulse of short duration is placed over the head.As a result, an electric current stimulating cerebral tissue is inducedinside the skull. In order to focus the magnetic stimulation on acertain selected area of the brain, it is often necessary to resort tothe magnetic resonance images taken from the test person's or patient'shead. Herein, the location and orientation of the coil acting as theresponse-evoking means of magnetic stimulation is determined in regardto the coordinates of the patient's head with the help of a suitablelocalizing system. Subsequently, the location of the coil can be mappedon the magnetic resonance images (MRI) of the patient, whereupon thesystem operator can readily focus the stimulation on a desired area. Onesuch method is described in FI Pat. Appl. 20021416.

Respectively, the magnetic resonance images of the patient's head areutilized when there is a need for locating a functional cerebral part inthe anatomy of the brain. Brain functions can be recorded and locatedusing conventional methods such as electro- and magnetoencephalography(EEG and MEG). Both of these methods typically use tens or even hundredsof measurement channels that sense electromagnetic fields evoked bybrain activity at different points about the head or on the scalp. Byknowing the exact locations of the measurement sensors relative to thehead coordinates, it becomes possible to identify brain functions and tovisualize the anatomical structure of each point in the magneticresonance images.

Conventionally, the head anatomy of the patient or test person beingexamined is first recorded by taking anatomical magnetic resonanceimages or other type of images resolving anatomical structures. Next, atleast three fixed marker points are selected on the head surface suchthat they can readily be identified on both the magnetic resonanceimages and the surface of the head. Advantageously, the auditorymeatuses and the nasion, for instance, are chosen to serve as markerpoints. As a result, a coordinate transformation can be formulatedsuitable for identification of a point in the magnetic resonance imagescorresponding to a certain point on the surface of the head. Thus, e.g.,the location of a TMS coil in regard to anatomical structures can beascertained or, alternatively, a stimulus-responsive point in the brainlocated with the help of MEG can be located in regard to anatomicalstructures. Various techniques are available for making a suitablecoordinate transformation. Magnetic resonance images of the testperson's head are required in the implementation of this method.

Using known methods, magnetic resonance images can be deformed so as toprovide correlation with the respective computer-aided tomographyimages. The deformation method is called image fusion. In this process,both ones of the sets of images are analyzed to find a plurality offixed marker points that are identifiable in both image sets.Subsequently, a deforming transformation can be carried out such thatthe corresponding points of the images to be matched become aligned witheach other.

In the art are also known methods for warping magnetic resonance imagestaken by MRI techniques from a test person so that the nonidealproperties of magnetic resonance imaging such as the nonlinearity ofgradient fields, are corrected. In these methods, correction factors aremeasured or computed and thereupon the images are respectively deformed.In EP patent publication 1 176 558 is further described a method forexternal patient contouring with the help of a suitable surface imagingsystem, whereupon the information thus gathered is used to deform thepatient's MRI images for planning a radiotherapy treatment.

Further methods known in the art are based on deforming by dilatationand contraction warping techniques a set of MRI images taken fromdifferent persons so that first the same fixed anatomical or functionalmarker points are identified in the images of each one of test personsindividually. Thereupon a mathematical mapping is computed individuallyfor each test person such that the test person's MRI images aretransformed in a fashion allowing the selected marker points of thedeformed image sets to have the same coordinates for all the testpersons. One such method is the so-called Talairach cerebral imagingsystem (J. Talairach and P. Tournoux, Co-planar Stereotaxic Atlas of theHuman Brain, New York, Thieme Medical Publishers, Inc., 1988). The goalof this system is to deform the MRI images of different persons so thatthe MRI images of the different persons' brains can be compared witheach other.

The above-described methods have in common that all of them needanatomical images of the test person's head.

Still further in the art are known methods in which the head contour isdetermined by means of an imaging system and the thus mapped surface ofthe head is formed into a triangulated grid that serves as amathematical model in the computation of electro-magnetic fieldsassociated with the use of MEG, TMS or EEG. Attempts have also been madeto determine by statistical methods from the head contour such atriangulated grid that further represents the brain contour of the sametest person. In this kind of method, magnetic resonance images areutilized to generate a statistical model representing correlationbetween the surfaces of the head contour and the brain contour. One suchmethod is disclosed in publication D. van't Ent, J. C. de Munck, andAmanda L. Kaas, A Fast Method to Derive Realistic BEM Models for E/MEGSource Reconstruction, IEEE Trans. Biomed. Eng. (2001), BME48(12):1434-1443. This method, however, is not used for processing MRimages.

A problem hampering the use of the prior-art methods and apparatuses isthat the analysis or visualization of data represented by the MRI imagesis possible only by taking the magnetic resonance images separately fromeach patient's or test person's head. Due to the high cost of magneticresonance images of the head, also the overall cost of TMS, EEG and MEGexaminations become high. Resultingly, the availability of TMS, MEG andEEG is limited.

If magnetic resonance images taken from the head of the person beingexamined are not available or the use thereof is not desirable, it isdifficult to visualize even coarsely the area of the skull hiding agiven brain region of interest. The basic reason hereto is that the headcontour and size vary largely from person to person.

In a typical TMS examination, for instance, it may be desirable to focusthe magnetic stimulus on the prefrontal region of the left hemisphere byplacing a figure-of-eight stimulation coil at the desired area of thehead. However, it is difficult to select the proper area on the head ifno anatomical images of the interior structures of the head areavailable. Respectively in a typical MEG and EEG recording session aresponse is discovered relating to a certain task, which can be locatedinside the head in relation to marker points situated external to thehead. Lacking access to anatomical images illustrating the interiorstructures of the head, however, it is difficult to tell the anatomicalpart of the brain that coincides with the identified point of response.

In another typical MEG or EEG examination, the task may be to identifybrain activity at two different regions of the brain as a response to,e.g., a task involving motor skills. In this exemplary case, the firstregion can be positively identified based on such variables, amongothers, as the characteristic waveform of the response to represent thefunction of the motor cortex, while the anatomical locus of the othercomponent of the response cannot be located without resorting tomagnetic resonance imaging or other techniques such as computer-aidedtomography suited for resolving anatomical structures.

It is an object of the present invention to provide an entirely novelkind of method capable of overcoming the problems of the above-describedprior art.

Accordingly, the invention strives to achieve a fully new approach tothe approximate localization of the major brain regions in a testperson's head without the need for magnetic resonance imaging. Themethod is particularly useful in the focusing of magnetic stimulationand in the interpretation and visualization of results obtained by meansof magnetic stimulation, EEG and MEG. This facility can be employed,e.g., for making screening measurements for large groups of patientswithout the need for taking costly MRI images from each patientindividually.

The goal of the invention is attained by virtue of modeling thecoordinates of the test person's head as to its different internalanatomical regions and particularly its different brain regions on thebasis of the test person's head contour and, additionally, on the basisof the different internal anatomical regions, particularly its differentbrain regions, actually recorded from the head of another test person.

More specifically, the method according to the invention ischaracterized by what is stated in the characterizing part of claim 1.

The invention offers significant benefits.

One major advantage is that the patient need not be subjected tomagnetic resonance imaging to identify the internal anatomy of thepatient's head for proper focusing of magnetic stimulation or analysisof MEG and EEG recordings.

Another advantage is that the method allows the stimulation responses ofdifferent patients to be compared with each other in the coordinatesystem of a “standard head”.

A third advantage is that a deforming image transform can be carried outor refined using functional marker points identified in the interiorvolume of the brain.

A still further advantage is that the method makes it possible toreadily indicate without anatomical imaging the coarse coordinates of apoint on the head surface under which a given anatomical brain region islocated. Furthermore a coarse location of a given brain anatomicalregion under a selected point on the head surface is possible withoutthe need for magnetic resonance imaging.

In the following the invention will be examined with the help ofexemplary embodiments and by making reference to the appended drawing inwhich

FIG. 1 is a schematic illustration showing the use of a method accordingto the invention in a single plane.

Referring to FIG. 1, the upper diagram shows a single plane of accuratemagnetic resonance images taken from the head of a test person B. Testperson A is measured only for the external dimensions of the head inorder to draw the sectional plane A shown in the middle diagram.According to the invention, the coordinate data of diagram B are dilatedand/or contracted (that is, scaled) so as to fit the data within theconfines of sectional plane A, whereby the diagram of sectional plane Ais transformed into a modeled sectional diagram A′. In the exemplarycase, it has been necessary to dilate the diagram shape of sectionalplane B in the vertical direction and to contract in the horizontaldirection.

The above-described procedure is applied entirely identically also inthe height direction, whereby three-dimensional modeling is attained.

Accordingly, the invention is based on using a method wherein the headcontour of the person (first person A) being examined is determined bymeasuring the coordinates of selected marker points on the scalp withthe help of a localization system. Advantageously, the number ofmeasured marker points is some tens and they are located at differentsides of the head. The greater the number of measured marker points thebetter is the result of the deformation process. Already five markerpoints on the head (forehead, left side, right side occipitalprotuberance and parietal top) give relatively accurate results. Next,the head contour of some other person (person B) is determined frommagnetic resonance images taken earlier from this person's head. Theimages of person B are deformed (scaled) computationally usingtranslation, rotation and linear and/or nonlinear deformation so as tomake the shape of the head images correlate with the contour of theperson's scalp, whereby a deforming linear or nonlinear transformationbetween both shapes takes place. The head images taken from person B mayalso be called a standard head. In the method the transformation isapplied volumetrically to the entire sets of magnetic resonance images,that is, also to coordinates located in the interior volume of the head.Herein the location and shape of the anatomical structures may becomedistorted. It is also possible to use a plurality of standard heads(e.g., separately for adults and children), whereby a standard head ofclosest fit can be individually selected for each patient. Also theracial differences between head contours can be taken into account bymaintaining a selection of different standard heads. Advantageously, thestandard head is computed using a set of MRI images having a goodresolution, e.g., 256×256 pixels in each sectional plane.

Image deformation (scaling) can be carried out, for instance, in thefollowing manner. First, the magnetic resonance images of the standardhead, that is, those taken from the head of person B, covering an entiresectional plane of the head are segmented by determining the coordinatesof selected marker points on the skull surface. Next, selected points ofthe scalp of person A being examined are determined in the earlierdescribed fashion using a localizing system. Thereupon a suitable linearor nonlinear deformation algorithm is carried out such that the magneticresonance images of person B are deformed maximally well to correlatewith the head shape of person A. The deformation transform may also beincomplete, whereby the shapes of the two heads are not aggressivelydeformed to full correlation. Suitable deformation algorithms areextensively described in the literature of the art. The magneticresonance images can be represented digitally in any known graphicformat such as pixel or vector graphics.

One exemplary deformation technique comprises determining from themagnetic resonance images of person B the location of, e.g., five markerpoints (the left and right auditory meatuses, the nasal bend also calledthe nasion, the occipital protuberance also called the inion and theparietal top). The respective marker points are determined with the helpof localizing system from the head surface of the person being examined.First, the marker points are registered with each other by way ofcarrying out a transformation that by translation and rotation makes therespective marker points of the heads of persons A and B to convergewith each other. Subsequently, a linear scaling algorithm can be appliedto the magnetic resonance images of test person B can be subjected tolinear scaling such that the respective marker points unite with eachother. As a result, a deforming transform takes place capable of makingthe head shape identified from the magnetic resonance images of person Bto correlate coarsely with the head shape of person A. When necessary,the procedure may be similarly extended to correlation of a largernumber of marker points.

One possible deformation procedure comprises the use of an algorithmdescribed in publication J. Lötjönen, et al.: Model Extraction fromMagnetic Resonance Volume Data Using the Deformable Pyramid, MedicalImage Analysis, Vol. 3, No. 4, pp. 387406, 1999). First, the magneticresonance images of person B are processed to determine marker points onthe head surface, e.g., by image thresholding. The head contour ofperson A is determined at N points using a localizing system. Both setsof points are registered with each other by way of performingtranslation and rotation operations such that make the sets of points toconverge with each other maximally well. When using optimally selectedtranslation and rotation operators, one possible strategy is, forinstance, to aim at a minimum sum of squares of differences betweenlocal radii of curvature on the correlating surfaces. Next, the magneticresonance images are divided into a cubic grid of 3×3×3 voxels. Anenergy function E is defined that may be, e.g., the sum of distancesfrom the points of image set A to the respective next closest point ofimage set B. Also for each elementary cube of the grid is written adeformation function f(x,y,z) that typically is a spline or polynomialfunction (such as Bernstein polynomials) and thus defines the amount oftranslation at other points of the cubic grid caused by a shift of onecorner point of the grid. Generally, the amount of grid deformationbecomes the smaller the larger the distance of the grid point from thecorner point. The deformation function may be linear or nonlinear. Next,the locations of the grid points are translated so as to minimize energyfunction E. As a result, the elementary cubes of the initially perfectcubic grid are dilated or contracted and thus deformed. The deformationfunction f is applied to each elementary cube. After the minimization ofenergy function E, the surfaces of the heads correlate with each other.In practice, also certain boundary conditions must be defined for thecubic grid. For instance, it may be advantageous to confine the dilationof the individual elementary cubes so that the dilation of allelementary cubes is uniform. Such a suitable boundary condition may beimplemented in energy function E.

In another embodiment of the invention, also functional marker points ofthe brain may be utilized. Herein, the localization of the motor cortexarea of a person being examined but not having MR images of the headavailable can be carried out by magnetic stimulation or, alternatively,using electroencephalography or magnetoencephalography or infraredtomography. Localization is performed relative to external marker pointsof the head (e.g., ears and nose). Similar localization is performed inbeforehand for another person (person B serving as a standard head) forwhom the magnetic resonance images of the head are available. Thelocalization of the motor cortex of person B is performed from the MRimages. The set of magnetic resonance images is warped so as to make thelocations of the motor cortex of the patient and the second person tocorrelate. Additionally, the MR images of person B are deformed so as tobring them into at least partial correlation with the head shape of theperson being examined. A similar procedure is also applicable to theutilization of multiple different functional marker points such as themotor or visual cortical areas of both hemispheres. When so desired, itis also possible to utilize herein the location of such a functionalmarker point that has been determined by statistical methods for aplurality of persons.

An example of the use of functional marker points in image deformationis represented by TMS. With the help of this method, the location of themotor cortex can be readily determined by moving the stimulation coilover the head until the strongest muscle response (recorded by EMG) isdetected in the hand muscles of the opposite side of the body. The samelocalization may be carried out for both hemispheres. Using suitableweights in the deformation procedure, the motor cortex locations of theperson being examined and person B are made to coincide with each other.

An essential feature of the present method is that the locations of themagnetic stimulation coil, the EEG electrodes or the MEG sensors aremeasured relative to the test person's head coordinates using alocalization system. Herein, on the person's head is mounted a positionsensor whose location can be determined with the help of a localizationsystem. The localization system is used for determining at least threemarker points of the head (such that may also be identified in the MRIimages), whereupon the deformation of the image coordinates can beperformed. The localization system used in the invention can be based,e.g., on infrared radiation or electromagnetic fields. This kind ofequipment is commercially marketed, e.g., by a Canadian company NorthernDigital Inc., for instance.

In the context of the present application, scaling refers to a dataprocessing method in which data generally representing an image istransformed into another form by linear or nonlinear procedures ofdilatation/contraction warping of the image. An alternative term forthis operation is deformation.

1. A method for modeling, with a processor, different functional areasof a brain within a second head to focus magnetic stimulation and/orvisualize the results of magnetic stimulation techniques,magnetoenecephalography (MEG) or electroencephalography (EEG), themethod comprising: a) determining the location of at least onefunctional area of a brain within a first head in three-dimensionalspace, b) determining the external dimensions of the second head, and c)scaling, with a processor, location data of said at least one functionalarea of said first head in three-dimensional space to correlate withsaid external dimensions of said second head, thereby defining thelocations of the at least one functional area in said second head suchthat the location data of the functional areas of the brain of saidsecond head are modeled without anatomical images of the internalstructures of said second head.
 2. The method of claim 1, furthercomprising focusing magnetic stimulation and/or visualization of resultsobtained by magnetic stimulation, MEG or EEG based on results of saidscaling location data.
 3. The method of claim 1, wherein said locationdata is displayed in an image format and the scaling thereof in step c)is implemented by mutual moving of individual pixels.
 4. The method ofclaim 1, wherein a response recorded by MEG or EEG or, alternatively, aneffective stimulating field of trans-cranial magnetic stimulation (TMS)is localized in relation to anatomical marker points determined on thesecond head surface.
 5. The method of claim 1, wherein said step b) ofdetermining the external head dimensions is performed by using infraredlight, electromagnetic fields, laser light or a pointer equipped withelectrical position sensor means.
 6. The method of claim 1, wherein saidstep a) of determining uses internal structures of a plurality of headsof persons of substantially the same age; said step c) of scaling usesan image scaling algorithm and includes adjusting the distance from thecortex to the scalp to a value typical for the persons of substantiallythe same age.
 7. The method of claim 1, wherein the step c) of scalingperforms a deformation operation utilizing location data of suchfunctional points of the brain that are localized solely with the helpof magnetic stimulation, MEG or EEG as functional points of the brain.8. The method of claim 1, wherein said step of scaling performs imagedeformation using a minimizing algorithm that minimizes the mutualdistances between the respective points of the deformed image of thesecond head and the points measured on the surface of a first head. 9.The method of claim 8, wherein the computation results of theminimization algorithm are accepted even when the mutual distancesbetween respective image points are not reduced to zero.
 10. The methodof claim 1, further comprising generating visual results of TMS, BEG orMEG examinations performed on a patient having no magnetic resonanceimages of his/her head available.
 11. The method of claim 1, furthercomprising displaying results in a single set of MR images obtained frommeasurements performed on a plurality of test persons.
 12. The method ofclaim 1, further comprising selecting, as a first head, a head from alibrary of plural magnetic resonance images taken from a plurality ofpersons representing heads of different types and shapes.
 13. The methodof claim 1, wherein scaling comprises linear scaling.
 14. The method ofclaim 1, wherein scaling comprises nonlinear scaling.
 15. The method ofclaim 1, wherein the method further comprises d) obtaining athree-dimensional image from magnetic resonance imaging orcomputer-aided tomography of the first head.
 16. The method of claim 1wherein the step b) only determines the external dimensions of thesecond head without directly determining the location of internalstructures of the second head in three dimensional space.
 17. The methodof claim 16 wherein the step b) is performed without acquiring orgenerating any information regarding the location of internal structuresof the second head.